We know this from the statistics of the relative few non-Christadelphians who later join the religion compared to the 7 billion people who never do. This makes one's family of birth the single most influential reason for Christadelphians believing what they do, with relatively few exceptions. The same also holds true for most of the world's religions, although the larger and more influential the religion, the more likely it is that someone born outside the religion might later join. But even for the world's largest religions, it is still more likely that someone born outside the religion will remain outside it than for that person to join the religion during their lifetime.
It might seem unusual to speak of the reasons for belief in this way, and that is because our brains are excellent at post-rationalisation. We tend to believe first, and then we seek explanations for the belief second. We then convince ourselves that the explanations we came up with were the original cause of the belief, even if they weren't. That does not mean the reasons and explanations we come up with are necessarily false or unsound, however. Such explanations may well provide sufficient justification for continuing to believe something even if they were not actually the reason why we arrived at the belief in the first place. We can still use our reasoning faculties to weigh up the various explanations to see which ones stack up to the evidence.
However, what we're not very good at is abandoning a belief if it turns out that it does not comport with reality. Thus, once people are entrenched in a belief system like religion (which usually happens from early childhood onwards), they are far more likely to double-down on their beliefs and deny any contrary evidence, than to accept the often-frightening possibility that one or more of their cherished beliefs may be less than certain. This is especially true if the belief(s) in question are tied to one's culture and even their core identity, and such is the case with religions like the Christadelphians.
Realising that we still believe many things we were taught when we were children, and that our entire worldview was moulded by the culture we grew up in, and that those who taught us were also influenced in the same way and via the same process when they grew up, we should start to become very skeptical about our ability to independently assess the things we were taught. Almost every belief we hold is backed by underlying assumptions that derive from our culture and upbringing, and even those assumptions are built on yet deeper assumptions, and so on.
Our default instinct should be to question all such received "truths", knowing the process by which they have been perpetuated. To question something does not mean to automatically reject it - but rather to be skeptical and open to the outcome going either way. But all too often people follow the much easier road of unquestioningly accepting what they were taught, as if, out of more than 7 billion people on earth, they happened to be so lucky as to be born into the correct religion, which in the case of Christadelphians is against staggering odds!
I will leave you with a thought experiment. This goes for both believers and non-believers alike.
- Think about why you believe, or don't believe. List the primary reasons (in your head is fine).
- Now change the question to ask what was the actual process you followed to arrive at the beliefs you hold. Think back to the events that led to your change in beliefs, or the formation of those beliefs. Can you see how that process influenced the position you now take?
- Now imagine you had been born into different circumstances. A different religion perhaps and/or a different country (those often go together anyway). How likely do you think it is that you would have arrived at the same beliefs? As a way to answer this question - ask how many people from that other religion actually do arrive at beliefs similar to yours now.
The thought experiment is intended to show how our beliefs are influenced by external factors, especially religion and culture. Obviously such influences do not automatically mean our beliefs must be false (otherwise known as a genetic fallacy), but they should make us highly skeptical of them, knowing that we most likely did not arrive at them via purely rational means.
Any honest pursuit of truth must therefore incorporate some method for detecting and correcting false beliefs, which includes being skeptical of our current beliefs and being prepared to re-evaluate and change our beliefs as new evidence comes to light.
"If you would be a real seeker after truth, you must at least once in your life doubt, as far as possible, all things."
- René Descartes